The invention relates to a method for distinguishing between interfering signals caused e.g. by air bubbles, existing as interfering volumina in a coupling means and signals generated by workpiece defects during the ultrasonic testing of workpieces according to the pulse-echo-method.
The German Application No. 27 04 128 describes a method for a dynamic distinction between disturbance signals coming from small reflectors, like floating scales or air bubbles, and the true faults in the test piece at the ultrasonic test with a relative motion to the transducer and defects, where the transit times between transmitter pulse and subsequently received signals are compared in order to determine whether this is due to a flaw in the workpiece or to an interfering volume existing in the coupling means. For such a distinction it is required that the workpiece is to be turned relatively to the transducer in order to obtain a relative motion between the two, which is not existing to an interfering volume in the coupling means.
The German Disclosure 29 16 938 describes a method and an apparatus for reducing virtual signals by ultasonic testing in immersion technique. In order to eliminate interfering signals to a far extent, the ultrasonic transducer is surrounded by a snorkel made of a sound-absorbing material and preferably extending up to the surface of the test piece. Hereby it is guaranteed that any interfering volumina present at the outside of the snorkel region cannot influence the test results. However, if air bubbles or other particles are present within the snorkel region, the interfering signals can still be produced by them and lead to wrong elevations. If a snorkel end is directly adjacent at the workpiece, the workpiece cannot be set in rotation like it is very often required in the ultrasonic testing. A falsification of testing results can also happen if the sound waves oscillate between the ultrasonic transducer and an interfering reflector for several times so that multiple echoes are generated which are falling within the evaluation range of true faults and thus are simulating apparent defects. With the method described above, these multiple echoes cannot be eliminated and therefore represent considerable disadvantages of the known art.